An X-ray imaging apparatus is equipped with an X-ray generator, an X-ray tube, an X-ray tube holder that holds the X-ray tube so as to be suspended from a ceiling or holds the X-ray tube above a floor, a collimator (X-ray diaphragm) that controls an X-ray irradiation area from the X-ray tube by a leaf, and an X-ray detector (an X-ray film, a CR (Computed Radiography) or an FPD (Flat Panel Detector)) that detects X-rays. A subject is held by using an X-ray imaging stand (a table or a stand), and X-ray imaging is performed by accommodating the aforementioned X-ray detector in the X-ray image capturing stand. In cases where the X-ray detector is an FPD, the X-ray image (X-ray captured image) captured from the FPD is transmitted to a digital device connected to the FPD and displayed on the monitor of the digital device (see, for example, Japanese Patent No. 5,344,807).
In the collimator that controls the X-ray irradiation area from the X-ray tube by a leaf, the irradiation field of the rectangular X-ray formed by the collimator can be adjusted in the length of the two sides by, for example, a longitudinal dimension adjustment knob and a lateral dimension adjustment knob provided on the panel surface of the collimator. However, doing this adjustment while monitoring the subject on the X-ray imaging stand by irradiating X-rays to the subject before the X-ray imaging results in an increased subject exposure.
Therefore, in order to prevent such exposure, the collimator normally has a built-in irradiation field lamp which becomes a light source and has a function of irradiating light instead of X-rays. It is possible to adjust the position and size of the irradiation field before imaging without irradiating X-rays by confirming the irradiation field of light adjusted so as to match the irradiation field of X-rays. The light irradiation start or irradiation termination is performed by a light irradiation button. For example, each time the light irradiation button is pressed, irradiation start and irradiation termination are alternately repeated.
An operator such as a technician adjusts the adjustment knob of the collimator to confirm the X-ray irradiation area corresponding to the imaging site of the subject by the irradiation field of light. After that, the operator instructs the subject not to move the body, leaves the examination room surrounded by a material (such as lead) that does not transmit X-rays, and presses the X-ray imaging button of the X-ray generator to perform the imaging.
In the meantime, an X-ray imaging apparatus equipped with an automatic exposure control (AEC: Auto Exposure Control) that automatically and optimally controls the exposure of X-ray imaging is widely used (see, for example, Japanese Patent No. 5344807, Japanese Unexamined Patent Application Publication No. 2008-117641 and Japanese Unexamined Patent Application Publication No. 2013-135390, the disclosure of each of these documents being incorporated by reference in their entirety). This automatic exposure control (AEC) is a control in which a part of the X-ray (transmitted X-ray) that has passed through a subject is detected in a specific detection range (hereinafter referred to as “photo pickup field”) and the detected X-ray is converted into an electric signal (X-ray output signal), and when the integrated value of the electric signal (X-ray output signal) reaches an appropriate value, the X-ray is interrupted. By performing this AEC, the integrated quantity (incident dose) of the transmitted X-ray irradiated to the X-ray detector is set to an appropriate value, which in turn can obtain a desired luminance signal (pixel value).
Normally, in an X-ray imaging apparatus equipped with an AEC, an exposure control detection unit (AEC detection unit) is installed on the subject side of a Bucky stand (imaging table) that holds an X-ray detector. There is a plurality of photo pickup fields of the installed AEC detection unit, and photo pickup fields suitable for the imaging site are selected for each imaging. For example, in FIG. 2 and FIG. 3 of Japanese Unexamined Patent Application Publication No. 2008-117641 (incorporated by reference in its entirety), there exist three photo pickup fields (photo pickup field right side, photo pickup field center, and photo pickup field left side). During the X-ray imaging, when the incident dose of the X-ray incident on the selected AEC photo pickup field becomes equal to or larger than a predetermined value, the X-ray is immediately cut off.
Also, in Japanese Patent No. 5,344,807 (incorporated by reference in its entirety), an X-ray output signal is controlled in proportion to the area (effective area) in which the X-ray irradiation area in the photo pickup field (AEC photo pickup field) of the AEC detection unit is set. Even when the X-ray dose is reduced by the collimator, by controlling the X-ray output signal in proportion to the effective area, the incident dose to the X-ray detector can always be kept constant.
When an X-ray detector represented by an FPD or the like also serves as an AEC detection unit, compared with image detection pixels, in the detection pixels in the AEC detection unit, the accumulation operation time becomes shortened by the amount of time for the dose detection operation. As a result, compared with an image detection pixel, in the detection pixel in the AEC detection unit, the pixel value becomes small, casing image quality deterioration. Under the circumstances, in Japanese Unexamined Patent Application Publication No. 2013-135390 (incorporated by reference in its entirety), it has been proposed that the pixel value at the detection pixel is corrected so as to become the same as the pixel value at the image detection pixel.